1. Field of the Invention
This invention relates to surface sizing compositions for paper making. More particularly, this invention relates to stable surface sizing compositions which can be used in surface sizing of paper sheeted under wide pH ranges, especially neutral conditions (pH 6-9).
2. Description of the Prior Art
Current procedures for paper sizing involve two principal methods: (1) "internal sizing" wherein the sizing agent is mixed with pulp and thereafter the mixture is formed into a sheet having a uniform distribution of fibers and sizing agent, and (2) "surface sizing" wherein a sizing agent is applied to the surface of an already formed paper. Surface sizing has some advantages over internal sizing. For example, it leads to substantial savings in sizing cost and to improvment in paper quality, e.g., good writability, because almost all of the sizing composition is retained on the surface of the treated paper. Surface sizing may soon replace internal sizing in popularity.
However, surface sizing has some difficulties. The sizing is made in a size press. The size press is continuously operated at a high speed to increase the productivity, and under a high pressure in order to apply a sizing solution to the surface of the paper as uniformly as possible. In such a sizing operation, the sizing solution suffers heat and mechanical shock which are generated during the operation. In addition, aluminum salts such as alum elute from the paper into the sizing solution. These salts have been used in the wet end of the sheeting step. These eluted salts have some adverse effect on the sizing solution, because they facilitate precipitation of materials from the sizing solution. Therefore, the sizing agent which is the main ingredient of the sizing solution should be stable to heat, mechanical shock and be able to resist the aluminum salts.
Heretofore, various kinds of natural and synthetic resins have been proposed and used as surface sizing compositions. Some typical examples of the resins are a modified rosin, modified petroleum resin, styrene-maleic anhydride copolymer and alkyl ketene dimer.
However, these resins are not satisfactory, although some of them have a few advantages. A sizing solution containing a conventional sizing composition gradually loses its solubilizing property toward the components because of the heat, mechanical shock and aluminum ions. This gradually results in the formation of scum. The presence of scum causes a decrease in the sizing effect of the sizing solution, and has an adverse effect on the appearance of the treated paper because of the deposition of the scum on the surface of paper. Therefore, the sizing with a conventional sizing composition requires such troublesome operations as strict pH control of the sizing solution, frequent renewal of the sizing solution and washing of the size press and pipes, and eventually conventional sizing compositions cannot be used in a continuous sizing operation.
Another disadvantage of the conventional sizing compositions is that sizing effects vary widely depending upon the pH conditions under which the paper to be surface-sized has been made at the sheeting step. For example, a modified rosin and modified petroleum resin have good sizing effects only on paper that has been made by sheeting under acidic conditions of pH 4.0 - 5.0. Alkyl ketene dimer has a good sizing effect only on paper that has been made under neutral conditions (slightly acidic to slightly alkaline). Styrene-maleic anhydride copolymer is applicable to paper sheeted under wide pH ranges, but has no satisfactory sizing effect on paper made under neutral conditions. A need, therefore, continues to exist for a surface sizing composition which has a good sizing effect on paper sheeted under wide pH ranges, and which is stable to mechanical shock, heat and alum.